Tips for gaining weight?

Why don't you want to give him anything he wants to eat? At this point, it's all about the calories. I've had this discussion with so many people, including my brother who kept pushing "nutrition". At this stage in their life it's all about the calories. Would you rather he eat ice cream and have pounds to spare or starve to death eating carrots and kale?

My mom went through something similar. I got her back up to 110 pounds from 80 pounds with adult nutrition drinks(sugar milk) and ice cream(frozen sugar milk). Grandma went through the same thing. The "professionals" wrote her off and said it was time for hospice house. I disagreed. I did the ice cream and nutrition drink thing for her too. She got over it. Both mom and grandma are both eating mostly normally again. I think a lot of elderly die simply because they starve themselves to death and no one steps in to push the calories. It's called the death spiral. They stop eating which makes them weak which makes them even less likely to eat which makes them even weaker. Break the spiral.

Give you dad anything he's willing to eat. At this point, there's no such thing as bad calories.

My mother has also lost a lot of weight since going into permanent care, and it has nothing to do with poor quality of food. She is in a premium facility with good food, good variety, cooked on premises by qualified chef.
Some days she can manage herself, most days she does not know what eating utensils are for. So staff, or one of us 3, try to feed her but without a lot of luck. We take her lots of things to tempt her, we do not worry about the empty calories, just giving her things she used to love. We find something she enjoys, but then the staff overdo it and she gets sick of whatever it is she is eating.
Most success has come from the common milkshake, strawberry flavour. Staff adds supplements, we get extra ice cream and malt. Once she starts she will not let go until she gas sucked the container dry. The problem is that she has forgotten how to suck, and often cannot guide the straw to her mouth.
Often she thinks she is being poisoned and refuses food, often she claims she is not hungry because she has just eaten.....wrong...., other times she has no money to pay the waitress.
Recently I was visiting her memory care unit, a number of residents were refusing their afternoon tea because they could not get to the bank, they had no money, someone had stolen their cheque book. Staff were getting frustrated, I asked for my tea and said I was shouting everyone as a Christmas treat. Not quite a stampede, but most residents then demanded their refreshments. Freebies always go down with oldies, must have something to do with the depression era they were born and raised.
Flowerpower567, I would not worry about empty caloried food. Just give your dad what he wants. Anything to get him into a regular eating pattern again. If he has been thrifty all his life, let him think he has won a hamper but he has to sample all the food before he can keep the basket. If alcohol is allowed, give him a beer everyday.

He's 86 and Alzheimer's is a terminal disease, at this point the benefits of eating healthy just don't apply. AgingCare has a couple of article you may find helpful

https://www.agingcare.com/articles/high-calorie-foods-for-seniors-168493.htm

https://www.agingcare.com/articles/pro-tips-sneaking-calories-nutrients-into-a-seniors-diet-208396.htm

Would he like some beer in the afternoon? Maybe bacon and scrambled eggs for breakfast? Grits with cheese? This is making me hungry.

Don’t worry so much. We have many posts about the problems of modern medicine keeping people’s bodies alive when their minds have gone, along with their quality of life. Perhaps the change of routine triggered your father’s loss of appetite and weight, rather than the shortcomings of the facility. Keep him happy, and let nature take it’s course. Age 86 and with dementia, just keeping him alive is not the main aim.

I think it's okay to focus on getting calories in and not to focus on nutrition.

I think your dad should eat whatever he wants. It can be both nutritious and have plenty calories. Also if he has any other illnesses like diabetes or hbp just watch the sugar and salt intake. But if he's hardly eating give him those shakes, add some other flavors like kale, carrots, bananas, etc. He'll like it and eat it.
I use the same shakes and add bananas and strawberries things light in sugar. Then a couple hours later give him a pb&j. He'll eat those all the time so I pair it with other foods he likes to help him eat. But it's a difficult decision to make cus their appetites changes often.
Hopen it works out for you.

If I make it to 85, I intend to eat or not eat whatever I want, and drink or not drink whatever I want. This will be clearly stated in writing for my POA.

After 85, I just want nature to take its course. I do not want to get to the point where my 93-yr-old mother and her aged, informed neighbors are. Yes, they are safe, but they have no quality of life.

I faced the same problems and decided to let dad eat what he wants even if it is often ice cream, junk food, etc. He always has porridge and toast and usually small suppers but not always. I am glad to hear from other answers that it sounds like I did the right thing.

There is something called "cachexia" when the muscles waste away and the person loses weight and cannot gain it back no matter how many calories they ingest.
Toward the end of my husband's life he lost almost 30% of his body weight and just wasted away. That's Alzheimer's!